I agree that buying links is good because every site needs traffic or it will go absolutely nowhere. Trust me I spent a year going nowhere And someone correct me if I'm wrong, but I believe that there is a condition to googles standpoint. If you are going to sell your links and they are not relevant, then you need to use a nofollow tag to prevent the googlebot from reading the nonrelevant adspace. Makes sense when you remember that googlebot scans your site for keywords, and it also applies a certain "weight" to different formats. Such as the words in your title and heading tags as opposed to a long paragraph of words in standard formatting. So a problem is created when a site sells their links without regard to whether it "fits in" with the content of their site. And if you sell links to gambling sites and other websites in categories that google considers below their ethics standard then they will drop your PR. I'm pretty sure they'll penalize you more for a non-related gambling or spyware product link than they would just a standard non-related link. But I could most certainly be wrong about that.